The present invention relates to a method of forming a microlens made of a thermoplastic resin on a solid-state imaging device, and more particularly to a microlens forming method which can form a microlens made of a thermoplastic resin on a solid-state imaging device highly accurately even with a fine pattern of 0.5 .mu.m or less.
As a method of forming a microlens on a solid-state imaging device with a relatively good position accuracy, it is known to directly form a lens made of a thermoplastic resin on the solid-state imaging device, thus obtaining a so-called on-chip lens. Various methods of forming such an on-chip lens made of a thermoplastic resin have been proposed. For example, in case of using a photoresist material for the thermoplastic resin, a negative type resist is used to obtain a high stability against light.
FIGS. 2A, 2B, 2C, and 2D show an example of the conventional method of forming a microlens. The forming method shown in FIG. 2 includes the steps of (a) laminating a negative type resist 2 and a positive type resist 3 in this order on a solid-state imaging device 1; (b) patterning the resists 2 and 3; (c) removing the positive type resist 3 as an upper layer; and (d) thermally deforming the negative type resist 2 on the solid-state imaging device 1 into a semispherical shape.
FIGS. 3A, 3B, and 3C show another example of the conventional method of forming a microlens. The forming method shown in FIG. 3 includes the steps of (i) patterning a positive type resist 3 on a negative type resist 2 formed on a solid-state imaging device 1; (ii) thermally deforming the patterned positive type resist 3 as an upper layer into a lens shape; and (iii) etching the resists 2 and 3 to transfer the lens shape of the positive type resist 3 as the upper layer to the negative type resist 2 as a lower layer.
According to a further known method, a microlens is formed by directly patterning the negative type resist rather than patterning the positive type resist as shown in FIG. 3 and then thermally deforming the patterned negative type resist into a semispherical shape.
However, in the case of patterning a negative type resist, an organic solvent is used as a developer, and a pattern of the negative type resist is formed by a crosslinking reaction. Accordingly, in a developing process, the pattern is swelled in the developer, so that the patterning with a high accuracy cannot be performed. As a result, it is difficult to form a fine pattern of about 0.5 .mu.and obtain a high resolution. For example, in case of patterning CMS (chloromethylated polystyrene) generally used as a far-ultraviolet ray negative type resist, a mixed solvent of xylene and MIBK is generally used as a developer. Also in this case, the pattern is swelled, and it is therefore difficult to form a pattern of even 1.0 .mu.m. In contrast, a negative type resist having a high resolution has recently been put on the market. In general, such a negative type resist having a high resolution is a surface curing type resist. Accordingly, in the subsequent step of thermally deforming the patterned resist into a desired lens shape, the desired lens shape cannot be obtained.
Further, in the case of patterning the laminate of the negative type resist 2 and the positive type resist 3 as shown in FIG. 2, the negative type resist 2 is undercut as shown by a dotted line in step (b) of FIG. 2B in the etching process. Accordingly, in the subsequent thermal deforming step, a desired radius of curvature of the lens cannot be obtained.
Also in the case of transferring the lens shape of the positive type resist as the upper layer to the negative type resist as the lower layer as shown in FIG. 3, it is difficult to transfer the lens shape of the positive type resist to the negative type resist with a good repeatability because of ununiformity of the etching or the like.
Thus, it is difficult to form a microlens uniformly and highly accurately in the prior art methods. In particular, a fine lens pattern of 0.5 .mu.m or less demanded by recent solid-stated imaging devices cannot be formed with a high accuracy in the prior art methods.